This paper suggests implications for researchable questions about mathematics teacher development. We ask the following: what is necessary for research that includes mathematics specialists or other hidden school-based roles within a project beyond classroom teachers and students? Mathematics specialists (e.g. mathematics coaches, mathematics teacher leaders) support teacher learning and development using different models of school-based professional development. They can play a critical role in connecting research and practice. However, research in mathematics education has regularly neglected to examine their role in studies of professional development. We suggest there are two needs for creating and examining researchable questions about such hidden players in studies of professional development: (1) defining practices and hidden players involved in systematic school change and (2) identifying the unit of analysis and scaling up professional development. We summarize existing studies and present considerations for future research.